1. Field of the Invention
The present invention relates to a current monitoring circuit which can be used advantageously in a de-icing circuit. The current monitoring circuit employs a DC isolating amplifier circuit that permits precise and accurate conversion of a first analog signal responsive to a bus voltage to a second analog signal, which second analog signal is intended for use with a current comparing circuit over a relatively large temperature range.
2. Description of the Prior Art
The accumulation of ice on aircraft wings and other structural members in flight is a danger that is well known. As used herein, the term "structural members" is intended to refer to any aircraft surface susceptible to icing during flight, including wings, stabilizers, engine inlets, rotors, and so forth. Attempts have been made since the earliest days of flight to overcome the problem of ice accumulation. While a variety of techniques have been proposed for removing ice from aircraft during flight, these techniques have had various drawbacks that have stimulated continued research activities.
One approach that has been used is so-called thermal deicing. In thermal de-icing, the leading edges, that is, the portions of the aircraft that meet and break the airstream impinging on the aircraft, are heated to prevent the formation of ice or to loosen accumulated ice. The loosened ice is blown from the structural members by the airstream passing over the aircraft.
In one form of thermal de-icing, heating is accomplished by placing a rubber boot or electrothermal pad(s), including heating elements, over the leading edge of the aircraft, or by incorporating the heating elements into the structural members of the aircraft. Electrical energy for each heating element is derived from a generating source driven by one or more of the aircraft engines. The electrical energy is intermittently or continuously supplied to provide heat sufficient to loosen accumulating ice.
When using electrothermal pads to achieve de-icing, the pads are preferably heated intermittently. By operating the pads intermittently, and sequencing the operation of the pads so that relatively few of the pads are operated during any particular time period, the power required to support the de-icer is significantly reduced.
To operate the pads sequentially, time-sequencers are typically employed in conjunction with an electrical source. Such timer sequencers or timer controllers are adapted to sequentially apply current for desired time intervals to various pads mounted on a structural member of an aircraft. That is, when used with aircraft, these de-icing pads function as electrical loads operatively coupled to the aircraft bus. Loss of an electrical load results in a drop of current delivered to the load, i.e. a drop in load current. As discussed in U.S. Pat. No. 4,814,931 to Kugelman et al. ("'931 patent"), timer-controllers have been utilized with measuring or indicating means for informing operating personnel when the amount of load current flowing through the aircraft bus has dropped below an acceptable level.
In the current comparing arrangement of '931 patent, a voltage, representative of load current, is determined from an aircraft bus using a shunt. This representative voltage is transmitted to a comparator and compared with a high reference voltage and a low reference voltage. The reference voltages are developed as a result of adjusting the set point of the comparator. During operation, the representative voltage is compared with the reference voltages, and if, for example, the representative voltage falls below the low reference voltage, the comparator sends an activating signal to a lamp, or a like indicating device. This technique is quite effective as long as the voltage of the bus remains stable. In particular, as long as the bus voltage is relatively steady, any significant drops in load current can be attributed to a loss in one or more of the de-icing loads.
On the other hand, for the condition in which either the bus voltage can drop or a de-icing load can be lost, the above-described current monitoring technique is deficient. Since the load current will drop when either the bus voltage drops or a de-icing load is lost, the cause of drop in load current cannot be specifically determined from a comparing arrangement such as that in the '931 patent. Desirably, a current monitor would be available that would indicate a drop in load current due to a loss in de-icing load rather than to a decrease in bus voltage.